Patterns of direct and indirect interactions prior to systemic shifts in ecological networks

Background/Question/Methods

Ecosystems have the capacity to go through sudden regime shifts from one seemingly stable state to another potentially less desirable state. Such transitions may occur locally --- affecting only a single part of the system --- or systemically, when many parts of the system go through a transition simultaneously.

With simulations, we mimic the dynamics of networks of interacting species and develop indicators that help us identify systems that are prone to experience systemic shifts.

Results/Conclusions

Modules of highly cross-correlated species appear prior to systemic shifts, pointing towards the species that will go through a transition simultaneously. Positively correlated species will shift in the same direction, while negatively correlated species will shift in opposite directions.

The reason for the appearance of those modules is twofold. First, positive and negative cross-correlations appear as a result of the combined effect of direct interactions and indirect effects mediated through other species within the system. Only species that facilitate each other have the potential to collapse simultaneously, while species that have a negative effect on each other will shift in opposite directions. Second, species near a transition point become more 'sensitive' to perturbations. This makes them not only slower when recovering from a direct perturbation, but also when recovering from the indirect effect of a perturbation in species that are directly or indirectly connected to them.